A conventional wideband power amplifier is described in an RCA manual entitled "RCA Linear Integrated Circuits", Technical Series IC-41, 1967, pages 263 to 266, and is shown in the circuit schematic diagram of FIG. 7. The power amplifier as shown has a difference stage (labelled "first stage") which receives an input voltage signal applied across two input terminals. The output signals of the collectors of transistors Q3 and Q4 of the difference stage are provided as inputs to a driver stage (labelled "second stage"), and a driver signal is further provided to the base of a driver transistor Q14.
The driver stage controls the operating point and overall gain of an output stage (labelled "third stage") having two bipolar transistors Q17 and Q18 connected in series in the same direction. Transistor Q17 has its collector connected to one terminal of a supply-voltage source and its emitter connected to the collector of transistor Q18, and the latter transistor has its emitter connected to the other terminal of the supply-voltage source. transistor Q18 is the negative output transistor.
In the conventional power amplifier shown in FIG. 7, the transistor Q14 of the driver stage drives inverting transistor Q15 and output transistor Q18 in parallel. The transistor Q15 has its emitter coupled to the other terminal of the supply-voltage source, and its base connected to the base of the output transistor Q18 over a common base lead. The collector of transistor Q15 is connected through a resistor R7 to the one terminal of the supply-voltage source, and is applied to the base of transistor Q16, which is in common collector configuration and provides its emitter output to the base of output transistor Q17. An output voltage is obtained from the junction point between the two series-connected output transistors Q17 and Q18. Negative feedback from the output stage to the input of the transistor Q14 is supplied by a resistor R15 which is connected to the junction point between the two output transistors Q17 and Q18.
The conventional power amplifier described above is disadvantageous in that it has a high open-loop gain. The desired loop gain is achieved by providing external negative feedback from the output stage to the input of the driver stage. Furthermore, a change in load resistance at the driver stage transistor changes not only the current through the two output stage transistors but also the output stage gain.